The present invention relates to crystals related to X-ray analysis.
In the prior art, acid phthalate crystals are used in, inter alia, a wavelength dispersive spectrometer for the diffraction of characteristic wavelengths from the low atomic number elements, there being employed as such acid phthalate crystals, acid phthalate salts of potassium, rubidium, thallium, and ammonium.
One of the more popular acid phthalate crystals is the rubidium acid phthalate (RAP) crystal, but these RAP crystals and at least some others suffer from a significant disadvantage, in that, after approximately one year of normal use in an X-ray spectrometer, which is known to the art, the diffracted intensity markedly drops off, commonly to a level that is only some 20% of the initial diffracted intensity value.
There has been reported in the literature (Fregerslev, "X-Ray Spectrometry", Vol. 6, No. 2, 1977) a deterioration of the reflectivity of the RAP crystal used for three years as an X-ray monochromator for sodium (PW 1220 spectrometer), such deterioration being to the extent that the crystal could no longer be used in the application.
It is believed that the above deterioration in the RAP crystal is due to surface deterioration of the crystal.
The article by Fregerslev proposes that the deteriorated crystal be carefully washed with distilled water using a chamois-leather, to restore the characteristics of the crystal, it being reported that such washing treatment results in the increase in the count rate back to 70% of that of the original crystal.
Thus, the acid phthalate crystals used in the prior art suffer from the described deterioration in properties and the techniques known to the art for restoring the properties of such crystals are relatively time-consuming and provide, at best, only a partial such restoration. A further disadvantage of the prior art acid phthalate crystals is that their gradual deterioration might be reflected in the analytical results obtained with such crystals, it, therefore, being necessary to compensate or correct for such deterioration, which is undesirable and time consuming, as well as introducing the possibility of significant error even where compensation or correction is done.
The present invention seeks to provide an improved acid phthalate crystal for use in X-ray and other applications, and, further, to overcome, or at least alleviate, the above described disadvantages of the prior art crystals.
Briefly described, the present invention comprises an acid phthalate crystal, e.g., rubidium acid phthalate or potassium acid phthalate, comprising at at least that portion of the crystal surface that is to be impinged by X-rays, a layer of metal or metalloid. It is possible that the entire crystal surface be covered by the metal or metalloid layer, although it is generally preferred that only the X-ray impinged surface area be covered.